Integrated biotechnological strategies combining Dark Fermentation (DF) and Anaerobic Digestion (AD) are gaining attention for sustainable waste management and energy recovery. This study focuses on the valorization of Construction and Demolition Waste (CDW), particularly the asbestos-containing fraction (ACW). The objectives areas follows: (i) the denaturation of chrysotile fibers via bioleaching using volatile fatty acids (VFAs) produced through DF, (ii) the conversion of DF supernatants into biomethane through AD, and (iii) the reuse of residual biomass to enhance process circularity.

DF involves several metabolic pathways, including acetate, butyrate, lactate, ethanol, and propionate fermentation (Sekoai et al., 2021), with acetate and butyrate dominating under mesophilic and thermophilic conditions. The fermentation is influenced by operational parameters such as pH, temperature, and food-to-microorganism (F/M) ratio (Pandey et al., 2022). DF at 35 °C led to the accumulation of 6.3 g/L butyric acid and 1.9 g/L lactic acid, while at 55 °C, lactic acid increased to 4.6 g/L. Other by-products such as ethanol and formic acid ranged from 0.5–1.5 g/L.

To stabilize the VFA-rich effluents, a second-stage AD process was carried out on three DF supernatants (S1, S2, S3) and their mixture (Smix, 1:1:1). Net methane yields ranged from 186 mL (S1) to 341 mL (S3). AD of Smix produced 259 mL CH₄ and 103 mL CO₂, with a methane yield of 0.38 L CH₄ g⁻¹ COD (Yeshanew et al., 2016). The complete process generated 4.9 L bio-H₂ and 19.7 L bio-CH₄ from 1.5 L of DF effluent (COD: 33.7 g/L).

Residual sludge (10.1 g TSS; 67.3% VSS) could be reused as inoculum, although periodic removal is required due to silica accumulation.

This integrated DF–AD system represents a sustainable route for converting hazardous ACW into valuable bioenergy, contributing to circular economy goals and EU environmental directives.